This invention relates generally to ammonium nitrate and, more particularly, to the phase stabilization of ammonium nitrate such as to facilitate the incorporation and use of such phase-stabilized ammonium nitrate in gas generant materials such as used in the inflation of automotive inflatable restraint airbag cushions and the like.
Gas generating materials are useful in a variety of different contexts. One significant use for such compositions is in the operation of automotive occupant safety restraints. For example, it is well known to protect a vehicle occupant using a cushion or bag, e.g., an “airbag cushion,” that is inflated or expanded with gas when a vehicle encounters sudden deceleration, such as in the event of a collision. In such systems, the airbag cushion is normally housed in an uninflated and folded condition to minimize space requirements. Such systems typically also include one or more crash sensors mounted on or to the frame or body of the vehicle to detect sudden decelerations of the vehicle and to electronically trigger activation of the system. Upon actuation of the system, the cushion begins to be inflated in a matter of no more than a few milliseconds with gas produced or supplied by a device commonly referred to as an “inflator.” In practice, such an airbag cushion is typically desirably deployed into a location within the vehicle between the occupant and certain parts of the vehicle interior, such as a door, steering wheel, instrument panel or the like, to prevent or avoid the occupant from forcibly striking such part(s) of the vehicle interior.
Gas generant compositions commonly utilized in the inflation of automotive inflatable restraint airbag cushions have previously most typically employed or been based on sodium azide. Such sodium azide-based compositions, upon initiation, normally produce or form nitrogen gas. While the use of sodium azide and certain other azide-based gas generant materials generally meets current industry specifications, guidelines and standards, such use may involve or raise potential concerns such as relating to the safe and effective handling, supply and ultimate disposal of such gas generant materials.
In view thereof, significant efforts have been directed to minimizing or avoiding the use of sodium azide or other azide-based gas generant components or materials in automotive airbag inflators. Through such efforts, various combinations of non-azide fuels and oxidizers have been proposed for use in gas generant compositions. These non-azide fuels are generally desirably less toxic to make and use, as compared to sodium azide, and may therefore be easier to dispose of and thus, at least in part, found more acceptable by the general public. Further, non-azide fuels composed of carbon, hydrogen, nitrogen and oxygen atoms typically yield all gaseous products upon combustion. As will be appreciated by those skilled in the art, fuel materials having a high content of nitrogen and hydrogen and a low content of carbon are generally attractive for use in such inflatable restraint applications due to the relatively high output of gas resulting therefrom (such as measured in terms of moles of gas produced per 100 grams of gas generant material).
It has previously been proposed to use ammonium nitrate (AN) as an oxidizer in propellant and gas generant formulations, such as gas generant formulations for generating large volumes of gas for use in the inflation of automotive inflatable restraint airbag cushions. In particular, ammonium nitrate advantageously has the potential to be used in gas generant formulations which provide or result in relatively high gas yields, e.g., inflation gas yields of up to 4 moles of gas per 100 grams of formulation. In addition, ammonium nitrate generally exhibits a high degree of chemical stability within typical temperature ranges that such formulations are designed for application including, for example, elevated temperature conditions such as might be encountered within the interior of a vehicle which has been left exposed in the sun.
Unfortunately, the incorporation and use of ammonium nitrate in pyrotechnic gas generant formulations has generally been subject to certain difficulties or limitations. For example, ammonium nitrate-containing pyrotechnic gas generant formulations have commonly been subject to one or more of the following shortcomings: low burn rates, burn rates exhibiting a high sensitivity to pressure, as well as to phase or other changes in crystalline structure such as may be associated with volumetric expansion such as may occur during temperature cycling over the normally expected or anticipated range of storage conditions, e.g., temperatures of about −40° C. to about 110° C. As will be appreciated, such changes of form or structure may result in physical degradation of such gas generant formulation forms such as when such gas generant formulation has been shaped or formed into tablets, wafers or other selected shape or form. In particular, ammonium nitrate is known to undergo temperature-dependent changes through five phase changes, i.e., from Phase I through Phase V, with an especially significant volume change of ammonium nitrate associated with the reversible Phase IV to Phase III transition. Furthermore, such changes, even when relatively minute, can strongly influence the physical properties of a corresponding gas generant material and, in turn, strongly affect the burn rate of the generant material. Unless checked, such changes in ammonium nitrate structure may result in such performance variations in the gas generant materials incorporating such ammonium nitrate as to render such gas generant materials unacceptable for typical inflatable restraint system applications.
It has been found that the phase change-induced degradation of pelletized ammonium nitrate-containing compositions can be mitigated if the humidity is kept extremely low, i.e., if the percent water vapor in the atmosphere is kept below 0.05 wt. %. Maintaining such low humidity levels, however, is generally impractical for most manufacturing situations; thus there is a need and a demand for gas generant compositions in which ammonium nitrate is phase-stabilized under more realistic or practical humidity conditions.
Because ammonium nitrate is prone to such undesirable phase-changes, it is known to stabilize ammonium nitrate to produce phase-stabilized ammonium nitrate (PSAN). In particular, ammonium nitrate has typically been phase-stabilized by admixture and/or reaction with minor amounts of additional chemical species. For example, U.S. Pat. No. 5,071,630 teaches stabilization with zinc oxide (ZnO), U.S. Pat. No. 5,641,938 teaches stabilization with potassium nitrate (KNO3), and U.S. Pat. No. 5,063,036 teaches stabilization with cupric oxide (CuO).
Unfortunately, the inclusion or presence of various of such stabilizer materials in various formulations have proven to result or produce certain undesired consequences. In particular and as further described in more detail below, work by the Applicant has identified disadvantages in a number of such formulations for the intended use of the formulation. For example, the inclusion or use of zinc oxide as such a stabilizer has been found to result or produce desired phase stabilization of the ammonium nitrate but only for a relative short duration of time. The inclusion or use of potassium nitrate as such a stabilizer has been found to be generally limited or restricted by a need to include such a stabilizer in a high relative amount (e.g., in an amount corresponding to 9 weight percent of the formulation) in order to provide desired phase stabilization. Unfortunately, potassium nitrate in such a relative amount has been found to undesirably produce or result in excessive fuming, i.e., generation of particulate matter, such as to make such use less desirable. The inclusion or use of cupric oxide as such a stabilizer has been found to be generally effective, e.g., provide sufficient stability, when used in a relative amount of only about 5 weight percent. Unfortunately, cupric oxide in such a relative amount has been found to undesirably produce or result in the resulting composition having a burning rate with a high dependence on pressure. Those skilled in the art will appreciate that a higher dependence of burning rate on pressure generally makes it more difficult to design devices with stable and predictable internal ballistics.
In view of the above, there is a continuing need and demand for a phase-stabilized ammonium nitrate that satisfies typical gas generant temperature cycling requirements while simultaneously minimizing or avoiding various of the undesirable properties associated with, attributable to or resulting from common ammonium nitrate phase stabilizing additives.